1. Field of the Invention
The present invention relates to a projection exposure apparatus and an exposure method and more particularly to a projection exposure apparatus and an exposure method of a step and repeat type used for manufacturing semiconductors, liquid crystal substrates, magnetic heads and the like by a lithographical process.
2. Related Background Art
In order to manufacture semiconductors, liquid crystal substrates by the lithographical process, a projection exposure apparatus of a step and repeat type known as a stepper is used which projects the pattern image of a mask of a photomask, a reticles or the like on a light sensitive substrate mounted on a stage through a projection optical system. With the projection exposure apparatus of a step and repeat type, a wafer stage on which a light sensitive substrate such as a wafer is mounted is moved to and set at a predetermined position, and exposure is repeated to transfer a mask pattern onto a wafer after the wafer stage has stopped.
In the projection exposure apparatus, it has recently been required that a movable stage carrying a wafer or the like be stopped at the required position more accurately at a higher efficiency. Generally, the wafer stage mechanism of the exposure apparatus comprises a supporting mechanism for a wafer including a stage and a driving mechanism for the stage and forms a specific vibration system.
FIG. 7 shows a conventional projection exposure apparatus of this type which will now be described briefly. Illuminating light L1 emitted from a light source (not shown) such as a mercury lamp is incident on a reticle 5 such as a photomask through an illuminating system (not shown). The reticle 5 is mounted on a reticle stage 6 and positioned so that the center of the reticle substantially coincides with the optical axis AX of a projection optical system 8. The illuminating light IL which has passed through the reticle 5 arrives at a wafer 10 through the optical system 8 and focuses, on the wafer 10, the pattern formed on the reticle 5. The wafer 10 is loaded on a wafer stage 11. The wafer stage 11 can make stepping movement two-dimensionally in a plane substantially perpendicular to the optical axis AX and is finely adjustable along the optical axis AX. The wafer stage 11 is moved by a motor 9. A wafer stage control system (WS control system) 15 outputs a signal S1 for controlling the motor 9 based on aimed position information from a host computer 16. The motor control signal S1 is inputted to the motor 9 through a digital-analog converter (DAC) 14 and a power amplifier 13. The WS control system 15 detects the position of the wafer stage 11 at a predetermined resolving power (for example, 0.01 .mu.m as a unit) by a wafer stage interferometer (WS interferometer 12). The WS control system 15 moves the wafer stage to the aimed position and performs exposure. The aimed position is obtained by a method disclosed in the specification of U.S. Pat. No. 4,780,617 or the like. Namely, the aimed position is obtained by measuring several shots on the wafer as sample shots by means of an alignment sensor and obtaining arrangement coordinates of each shot by the use of statistical arithmetic analysis.
If the positional accuracy of only the wafer stage increases when the wafer stage is moved to the aimed position in the conventional technology, positioning reproduction accuracy and throughput are likely to be lowered.
Namely, when fine-positioning is made, dynamic characteristic factors such as fluctuation of the wafer stage and play of bolts are likely to vary. Thus, it is difficult to improve positioning reproduction accuracy per shot. For this reason, it is difficult to enhance the positioning reproduction accuracy for only the wafer stage.
Damped oscillation occurs to the wafer stage by an inertia produced by the speed reduction and stoppage of the wafer stage. Thus, exposure cannot be performed until the oscillation is attenuated fully and the wafer stage stops completely, and waiting time for attenuation lowers throughput.
If response of the wafer stage to the motor is improved in order to increase the positioning reproduction accuracy, the damping-force for the oscillation becomes insufficient. Thus, the waiting time for oscillation attenuation increases and the throughput is lowered.
There has been proposed a method for servo-controlling only the mask stage and the wafer stage by using the same length measuring system for measuring the positions of the mask stage and the wafer stage. When, however, deviation between the mask stage and the wafer stage is large, the servo control cannot be performed until the oscillation of the wafer stage is attenuated fully because of the limitation of the driving stroke of the mask stage and the response speed.
In the projection exposure apparatus used for manufacturing semiconductors or the liquid crystal display devices and so forth, especially when forming a circuit pattern on the second and subsequent layers on a wafer (or a glass plate, etc.), it is required that a reticle be aligned with the wafer at a high accuracy. A global alignment conceived as a typical alignment operation involves a step of measuring a position of the wafer (or a relative position of the wafer to the reticle) by use of an alignment sensor and a step of positioning the wafer on the basis of a measured result given by the alignment sensor and exposing it. Even when the alignment sensor performs a correct measurement, it follows that a positional deviation between the reticle and the wafer is caused if positioning of the wafer in a target position can not be effected exactly on the occasion of positioning of the wafer in an exposing position.
Generally, after measuring a position of each shot area on the wafer by, e.g., a TTL (Through The Lens) method or an off-axis alignment sensor, and when positioning of an exposing target shot area in the exposing position is effected, a laser interferometer monitors a position of a wafer stage in a state where the reticle is made static. Then, it is confirmed that the wafer stage exists within a given allowable value with respect to the target position, and, thereafter, the exposure is started. Specifically, for a predetermined fixed time, there is checked whether or not a deviation from the target position of the wafer stage (this may also be conceived as a relative position to the reticle) enters a predetermined window (two-dimensional allowable zone). For example, this is determined by knowing whether or not the deviation falls within .+-.40 nm for a time of 20 ms.
Further, in a projection exposure apparatus equipped with an alignment sensor base on a TTR (Through The Reticle) method, which is capable of directly viewing the relative position of the wafer to the reticle through the reticle and a projection optical system, the TTR alignment sensor detects a deviation in the relative position of the wafer to the reticle, and it is confirmed that the deviation falls within a given allowable value with respect to a target value for a fixed time as in the case of the laser interferometer. Thereafter, the exposure is started.
As explained above, according to the prior art, the exposure is not started if the deviation in the relative position between the reticle and the wafer does not fall within a 100% allowable value for the fixed time. For this reason, if mechanical vibrations are applied to the whole projection exposure apparatus, or if hunting of the stage system is produced, arise problems in which the exposure can not be started for a long time, or the positioning process is remarkably time-consuming.
To avoid this, for instance, even if the positioning allowable value is set larger so as not to reduce the throughput (the number of wafers processed per unit time), the deviation in the relative position between the reticle and the wafer does not invariably converge at a target range during the exposure under a great-vibration environment. It can be also considered that the wafer is located in a deviated position as the case may be.